OK from the wealth of info on the internet and with much experimentation I've developed a PWM modulator that will replace any modulation transformer and associated high power audio amp that's 'normally' needed to modulate an A.M. carrier up to 500W (50V 10A MAX!). Will run from a 12V battery or even 130V + if you use different (off board) FET's. The circuit is all on a PCB (screened, masked etc) and less than 50mm square.

I also have a 100W class E (CMCD or E/F with different O/P config) amplifier too no bigger than your finger as a suitable companion for the modulator.

Next couple of kits under development are an optional audio compressor for the PWM Modj and a PIC/DSS frequency source with 16 programmable channels

Both current kits on EBay;

A.M. Transmitter PWM Modulator Kit

Class E/D RF amplifier Kit 100W

Let me know if your interested as I'll give you a better price if ordered direct!